1. Field of the Invention
The present invention relates to a shaft for connecting to an impeller.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Turbocharger impellers are typically made of aluminium alloys to provide low rotational inertia with reasonable strength at a commercially-acceptable cost. Attachment of the impeller to the steel turbocharger shaft is achieved in various ways. For example, because of the relative weakness of aluminium and the small diameter of the shaft, one option is to provide the impeller with a steel insert containing a screw-threaded socket which can be threaded on to the shaft. This arrangement can take a higher torque than a connection in which the shaft is directly threaded into the aluminium body (the torque is proportional to the power transmitted across the joint, and so the impeller can be used at a higher pressure ratio than one in which there is a direct threaded connection).
Typically, such an insert is fitted into the impeller by shrink fitting; the aluminium body of the impeller is heated to expand the bore which is to receive the steel insert, while the insert is cooled, for example using liquid nitrogen, before being inserted into the bore. The resultant interference connection is restricted by the temperature to which the aluminium can be heated before its material properties are affected, and by the temperature to which the steel can be cooled.
EP1394387 describes an insert of this type, and further proposes an outer steel constraining ring which reinforces the frictional contact between aluminium impeller and the insert. Since the ring does not expand as much as the impeller body as the turbocharger heats up, the point of grip between the impeller and the insert remains within the axial extent of the ring during the whole operating cycle of the turbocharger, thereby preventing a tendency of the impeller to “walk” along the insert. As a consequence, the operating life of the turbocharger can be considerably extended in comparison with the conventional turbocharger without the constraining ring.
When a shaft is threadingly connected to an impeller (e.g. via an insert as discussed above), the threads on the shaft and the impeller may be self-tightening to provide a rotationally fixed connection between the impeller and the shaft.
Another option is for the impeller and the shaft to have corresponding abutment surfaces which engage when the impeller is screwed on to the shaft, thereby tightening the threads to provide the rotationally fixed connection. In such an arrangement the threaded portion of the shaft is typically axially spaced from the abutment surface of the shaft. Differential thermal expansion effects operating between the impeller and the shaft may then cause a differential thermal strain to be set up between the shaft's threaded portion and abutment surface. This strain can produce an undesirable level of stress in the threads of the shaft and impeller.